The safety of long-term steroid usage by women for purposes of estrogen replacement therapy (ERT) or oral contraception (OC) is currently under scrutiny. Epidemiological data can be hard to interpret due to changing patterns of usage and drug formulations. However, based on recent reviews by the WHI and others [1-6], we can at least estimate the magnitude of the problem. Analysis of the increased risk factor for breast cancer for OC users would be about 1.2±0.2, and for ERT users about 1.5±0.2. Using data from the USA, if the median age of OC users is 25 years, the very low rate of cancer incidence in that age group combined with the low risk factor implies very few additional cases per 100,000 women. Given the uncertainty in the risk data and the perceived social benefit, this is generally considered to be a small but acceptable risk [for an different view of OC risk using higher risk factors, however, see ref 7]. For ERT users, their median age of ca. 50 yr has a much higher cancer incidence which, when coupled with the increased risk factor, suggests that there would be about 100-150 additional cases per 100,000 women. This is a significant number of cases, and after review of the epidemiological data, current medical thinking in Canada and the U.S.A. is summarized in a set of recommendations given by the Canadian Task Force on Preventive Health Care in May, 2004 [6]:
Recommendations                1) Given the balance of harms and benefits, the Canadian Task Force on Preventive Health Care recommends against the use of combined estrogen-progestin therapy and estrogen-only therapy for the primary prevention of chronic diseases in menopausal women (grade D recommendation);        2) For women who wish to alleviate menopausal symptoms using hormone replacement therapy (HRT), a discussion between the woman and her physician about the potential benefits and risks of HRT is warranted.        
These recommendations raised warning flags for patients and physicians alike, due to the strong demand for hormone replacement coupled with the puzzling risk/benefit analysis implicit in the statement. Of course, a preferable alternative to traditional drugs used as hormone supplements would be a safer, non-carcinogenic compound.
Etiology of Breast Cancer: The etiology of breast cancer is complex, with hormone-dependent and hormone-independent components [3]. It was originally thought that the only relation between estrogens and cancer was through their ability to stimulate abnormal cell proliferation via estrogen-receptor mediated processes [see ref 8 and references therein]. However, as a result of new evidence on the relationship between estrogens and cancer the field is undergoing a “paradigm shift”. A new mechanism of interest, which involves the formation of catechol estrogens as metabolites and their subsequent oxidation to carcinogenic quinones, is not yet considered proven to be the dominant cause of breast cancer, but an increasing amount of evidence in its favor is appearing [8-17].
Quinone Formation and Carcinogenesis: The naturally occurring estrogens estradiol and estrone have the classic steroid structure containing the A, B, C and D-rings, where estradiol is shown in FIG. 1. The B, C and D rings are saturated, but the A-ring is an aromatic phenol. Phenols are easily metabolized in the liver and elsewhere by the enzyme cytochrome P 450 hydroxylase [22]. This leads to hydroxyl substitution at the positions adjacent to the first hydroxyl group (situated at position 3 in the A-ring), forming 2-OH estradiol and 4-OH estradiol. These metabolites, termed the “catechol estrogens” [9] can be further metabolized by oxidizing substances present in the cell, e.g. peroxidase/P450 or tyrosinase/O2 [23], or even in the presence of oxygen, to give the 2,3-quinone and the 3,4-quinone [9-17].
Quinones in general are electrophilic compounds which have a tendency to be tumor initiators and promoters, and several such mechanisms are known [22]. They can damage DNA by combining with nucleic acid bases thus causing replication errors [22]. They can deplete essential cellular antioxidants such as glutathione and thiol-containing proteins, subjecting the cell to oxidative stress [22]. They can act directly as free radical generators via reduction to the semiquinone form and subsequent redox cycling, producing superoxide ion [24]. Different quinones show differing amounts of cytotoxicity due to these competing mechanisms; some, such as the naphthoquinones and anthraquinones are highly cytotoxic [22].
FIG. 1 shows the biological scheme of quinone formation, starting from the natural hormone 17β-estradiol (hereafter “estradiol”). Here the quinone formed involves only the A-ring, i.e., it is a benzoquinone. In the case of one of the conjugated equine estrogens present in the ERT drug Premarin (currently the third most prescribed drug in the USA), the naphthoquinone was formed and it was shown that hamsters treated with the naphthoquinone for 9 months showed 100% tumor incidence [11,12]. This led Bolton, Cavalieri and co-workers to the conclusion that “metabolism of estrogens to catechols and further oxidation to highly reactive o-quinones could play a major role in induction of DNA damage leading to initiation of the carcinogenic process” [8,13,15-17]. This short summary describes the catechol-estrogen hypothesis of the etiology of breast cancer.
ERβ-Selective Agonists: The recent discovery that estrogens bind similarly to the two receptor subtypes, ERα and ERβ, and that these receptors have different tissue distributions, has resulted in major efforts to develop ligands which are selective agonists for either receptor [32,36 and 37]. Such compounds have considerable potential for the treatment of a number of symptoms and/or diseases associated with estrogen deficiency, including hot flashes, osteoporosis and cardiovascular problems [37]. Tamoxifen, and raloxifene, although developed before the discovery of the ERβ subtype, are now classified as selective estrogen receptor modulators (SERMs) [32]. Thus, tamoxifen and raloxifene exhibit both estrogenic and anti-estrogenic activity depending on tissue type. The anti-estrogenic activity of both compounds has been exploited to prevent re-occurrence of ER-positive breast cancer and the prevention of breast cancer in high risk women. Both drugs act as an agonist in the bone and thereby help prevent osteoporosis [37]. Tamoxifen, but not raloxifene, also acts as an agonist in the uterus and leads to an increased risk of endometrial cancer. However, neither of these compounds relieves hot flashes, the most common menopausal symptom.
In their search for ERα and ERβ-selective agonists most research groups have targeted non-steroidal families of compounds, inspired by the natural product lead structure genistein. Considerable success has been achieved in this area for structures illustrated, for example, by WAY 202196 and ERB-041 [38 and 39]. These compounds show not only strong binding but also excellent selectivity for ERβ vs. ERα; the binding affinity ERβ/ERα ratios for these structures are: genisten (41); WAY 202196 (78); ERB-041 (226). However, the latter highly ERβ-selective compounds appear to be devoid of classical estrogenic activity. They do not promote the growth of estrogen-dependent MCF-7 breast cancer cells, but also do not relieve hot flashes or protect against osteoporosis. The value of genistein for the treatment of hot flashes has not been unambiguously established, although soy products (containing genistein) are commonly used for this purpose.
There remains a need for estrogenic compounds that avoid the problem of quinone formation, while retaining hormonal activity. Compounds found to have such activity will be useful, for example, in hormone replacement therapy (HRT/ERT), in estrogenic hormone therapies and as contraceptives.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.